1. Field of the Invention
The present general inventive concept relates to an image display apparatus capable of producing an image in free space, and more particularly, to an image display apparatus having a structure to produce an erect real image in free space.
2. Description of the Related Art
Generally, an image display apparatus displays an image of an object on a two-dimensional (2D) plane, i.e., a screen. Such images displayed on the 2D plane by the image display apparatus are not natural. In an attempt to overcome this problem, a variety of image display apparatuses for producing an image in free space have been developed.
An example of a conventional image display apparatus is disclosed in U.S. Pat. No. 4,671,625, entitled “Optical Apparatus for Producing a Natural, Viewable and Optically Interactive Image In Free Space” and patented on Jun. 9, 1987, which relates to an optical apparatus capable of producing an image in free space.
Referring to FIG. 1, the conventional image display apparatus includes an elongated tubular shroud 3 of a rectangular cross-section closed at one end by a cathode ray tube monitor 1. An optical system is provided within the shroud 3. The shroud 3 is made of opaque material and is provided with a non-reflective internal surface. The optical system deposed within the shroud includes a first convex lens 5 and a second convex lens 7, each having an f-number between about 0.5 and about 1.5. The first and second convex lenses 5 and 7 are both Fresnel lenses.
An image 13 of an object 11 displayed on the cathode ray tube monitor 1 is produced in front of the second convex lens 7 via the first and second convex lenses 5 and 7. Accordingly, a viewer 10 can see the image 13 floating in free space between the viewer 10 and the second convex lens 7.
However, the conventional image display apparatus provides an image that is not upright but upside down (i.e., left inverted). In addition, when the first and second convex lenses 5 and 7 have the f-number between about 0.5 and about 1.5, the distance between the object 11 and the second convex lens 7 must be equal to or greater than the length of a cross-section of the object 11. Accordingly, the conventional image display apparatus is long and it is difficult to make the conventional image display apparatus compact. Moreover, when a small lens having an f-number of about 0.5 is used, distortion and chromatic aberration may cause problems in the displayed image.